The majority of electronic watches with analog display which are presently in existence are watches in which the indicating organs are driven by a single electromagnetic motor of the bipolar monophase stepping type which is conceived to rotate normally only in one sense by steps of 180.degree. in response to alternating polarity pulses.
At a certain epoch motors of this type comprised a rotor with a permanent magnet generally cylindrical in form and diametrally magnetized, a stator formed of two polar parts constituted by two distinct pieces situated in a single plane and which exhibited polar faces in the form of portions of a cylinder in order to surround the rotor and which were separated on either side of the latter by narrow slots, and a winding surrounding a core to which the pole pieces of the stator were fixed.
Furthermore the pole pieces of the stator were arranged in a manner such that their polar faces were slightly offset, in the contrary sense, relative to the axis of rotation of the rotor in order to create for such rotor two well-defined stable rest positions angularly separated by 180.degree. and in which its magnetization axis formed an angle of approximately 45.degree. with the axis which joined the slots of separation between such pieces.
These motors exhibited in fact serious disadvantages.
Effectively, in order to have them function in a sure manner with an optimum yield, it was necessary that the excentricity of the polar faces of their pole pieces be regulated with great precision.
To accomplish this, it was necessary initially to mount the motors on their supports, for instance base plates, which were provided with studs around which the pole pieces could pivot, next effect the regulation by means of excentrics with which they were provided and with the help of special tools, and finally fix these pole pieces on to the supports by means of screws. Such regulation thus took up a great deal of time. Furthermore, it was necessary to conceive watch movements in a manner such that one could see the parts of the pole pieces situated in the neighbourhood of the rotors and have access to the excentrics and to the blocking screws, thus rendering the bridges which permitted maintaining the rotors in place practically incapable of serving for other purposes. Finally, it happened that one was obliged to again effect the same operations of adjustment when it was necessary to disassemble the watches or when the latter had been subjected to shocks.
For this reason, such motors were used only during a very short period of time and have yielded place to those much easier to manufacture and to employ which are presently found in watches.
These present motors assume essentially three forms and are distinguished in fact from those which have just been discussed only in respect of their stators.
The simplest and by far the most current are those which possess a stator which is formed of a single piece cut out from a plate of soft magnetic material and which includes in its central portion a cavity bounded by an internal wall of essentially cylindrical form in which is coaxially placed a rotor. In these motors the polar parts are coupled between themselves by necks, i.e. narrow portions easily and rapidly saturable by the magnetic flux, which play practically the same role as the slots between the pole pieces of the previous motors and the excentricity of these pole pieces is replaced by notches or diametrally opposed flattened portions exhibited by the internal wall of the stator.
In the other two forms of bipolar monophase motors which one encounters in certain watches, but far more rarely, one again finds the idea of two distinct pole pieces.
In one case these pole pieces exhibit the same offset pole faces as those of the older motors mentioned hereinabove, but they are in this case coupled and welded between themselves on either side of the rotor by interposed elements of non-magnetic material, or practically in a manner to form a monoblock stator.
In the other case, these pole pieces continue to be separated by slots, but their polar faces in the form of portions of a cylinder are no longer offset and are provided with notches in order to define the rest positions of the rotor, and the motor includes in addition a cylindrical sleeve of non-magnetic material, fixed to a base plate on which the motor is mounted, which coaxially surrounds the rotor and which serves as a positioning and support element for the polar faces of the pole pieces.
Whatever be the manner in which they are obtained, these monophase bipolar motors have the advantage of being simple, occupying little volume and of low cost of manufacture, but they also show a serious defect.
When they are normally used in watches, i.e. when one is satisfied to have them turn in the sense for which they are designed and which permits advancement of the indicating means, for modifying the display information and above all for time setting, one is obliged to provide either mechanical arrangements which enable a correction of the time indication in both the advancing and retarding senses, or electronic arrangements which assure only a unidirectional correction. In the second case, the motor is, during a correction, driven by pulses of a fixed frequency or dependent from the speed of operation of a manual control element which is clearly greater than that of the pulses which it receives in the normal mode of operation. At the same time, in order that it may respond to each of the correction pulses, this frequency must remain sufficiently low, generally not exceeding 32 or 64 Hz, from whence the time taken to achieve correction may be quite long and not acceptable for watches having a seconds hand.
The oldest solutions which have effectively been put into practice and which have permitted to resolve this problem at least partially, are two in number.
The first consists of keeping the same bipolar monophase stepping motor and causing it to turn backwards as well by controlling it in a special fashion. Thus, one continues to benefit from the advantages of this type of motor, but the performances in the reversed direction are very mediocre relative to those obtained in the normal forward operation.
The second solution is to use a bidirectional motor such as that forming the object of U.S. Pat. No. 4,371,821 and which does not exhibit this difficulty.
This motor which also rotates by steps of 180.degree. comprises the same rotor as a monophase motor, a stator in a single piece with three pole pieces connected among themselves by necks and which itself also presents an essentially cylindrical cavity for housing the rotor and notches in the internal wall surrounding such cavity in order to define two rest positions at 180.degree. from one another for this rotor, and two windings surrounding two cores which couple mechanically and magnetically one of the pole pieces respectively to the other two. Such a motor is clearly of greater volume and of a higher cost of manufacture than a monophase motor.
These two solutions enable effecting electronically time setting and, for instance, far more rapid time zone changes in watches which comprise only minutes and hours hands, but unfortunately they are not yet sufficient to be applied to watches having a seconds hands and/or a date indicator.
This is one of the reasons for which there has begun some time ago the production of watches with two motors for driving the indicating organs or groups of indicating organs separately.
Another reason is the sparing of energy thereby achieved. This is the case in particular for a watch in which the two motors serve to drive on the one hand the seconds and minutes hands and on the other hand an hours hand and a date indicator, or for a watch in which the seconds, minutes and hours hands are driven by a first motor and a date disc by a second motor, but for which one has been obliged to forego time and date setting electronically.
A third reason is that one may thus easily increase the number of functions which the watch may fulfil, as for instance chronograph, alarm, etc.
Finally, a fourth reason is that it then becomes possible to reduce the number of parts in the wheel train of the watch.
Presently, to profit still further from all the advantages which the fact of not being limited to a single motor presents, one arrives at manufacturing watches which comprise four and even sometimes five thereof.
The difficulty is evidently that, for instance two motors cost twice as much and occupy twice as much volume in a watch as a single motor, or practically, if they are not completely identical.
This explains that one very quickly came to the idea of conceiving motors with a common stator for two rotors or even more.
This much said, at least since the epoch of the bipolar monophase motors discussed at the beginning, one generally has been used to manufacturing flat stators or the pole pieces which go to make them up in starting from a plate of soft magnetic material, in giving to them the desired form by simple or successive blanking, in placing them next in bulk into holders and in heating them thus in ovens in order to subject them to an annealing operation which enables realigning the crystals of the material of which they are formed, suppressing internal mechanical stresses etc.; otherwise said, restoring to this material the good magnetic properties which it had more or less lost by the deformation at the moment of the blanking operation.
Now this manner of operation gives rise to a problem. Effectively, flat stators or their pole pieces always have relatively fragile portions. This is the case in particular of the horns which the pole pieces must exhibit in order to be able more or less to surround a rotor, or the necks of single piece stators. Consequently, at the moment when these stators or these pole pieces are placed in bulk in the holders and emptied therefrom, or even simply when they are shifted during their transport, there are always certain which are deformed or break and which are no longer usable, this leading to an increase in the cost price of the motors which one forms with those which remain intact.
Furthermore, it is well evident that the more the stators and the number of fragile portions which they comprise become larger, the more the risks of deformations and breakage increase, and the greater the loss with each rejected piece, taking into account that their cost of manufacture and very often the investments necessary for their development are also ever greater.
Thus, for stators of motors having two rotors which one is beginning to provide this problem becomes important, above all if the arrangement is such that the rotors are very close to one another in order to reduce as far as possible the number of wheel sets of the watches provided with this type of motor.
To solve this problem, it is sufficient to recall a solution which has been found for eliminating the difficulties of regulation of monophase motors having a two-piece stator previously discussed, and which enabled at the same time to eliminate the problem for such motors.
This solution which formed the object of patent GB 1 457 676 and which has been employed during two or three years consisted initially of cutting out from a plate of soft magnetic material a piece with cavity, the interior and the exterior contours of which corresponded exactly to the form and the relative position of the pole pieces of the stator which one wished to obtain. This piece was then entirely welded on to a support plate of nonmagnetic material, the exterior contour of which corresponded in large measure to its own in order to permit the correct positioning thereof before the welding and which exhibited a round hole, the circumference of which circumscribed the wall of its cavity. After this one proceeded to a further blanking operation to obtain the slots which were to separate the two pole pieces and one heated the assembly in order to subject the stator to an annealing operation. Naturally, in order that the pole pieces during this annealing and at the moment of cooling down not be subject to stresses and deformations, and in order that they remain correctly positioned relative to one another, it was necessary to choose for the support plate a material which had the same coefficient of thermal expansion or almost the same as that of the stator.
Relative to what has previously been done, this support plate thus also had the effect of reinforcing the pole pieces of the stator and assuring a perfect protection of the latter.
Furthermore, it is entirely possible to join such a plate to any flat stator whatsoever in order that it play likewise this role therefor and often in limiting its extent in order that it cover only a zone in which are found the fragile portions of such stator.
At the same time, if one were satisfied to operate in this manner, one would encounter two other problems.
The first is that if it is true that the presence of reinforcing plates may effectively permit a considerable reduction in the cost price of large stators by eliminating practically entirely the risk of having rejects following the annealing and in spite of the cost of these plates and their attachment to such stators or the stators to them, this is much less the case for small dimension stators or the pole pieces thereof which are, even so, less fragile and the manufacture of which now is clearly less expensive. In this case providing reinforcing plates in order to avoid losing a certain percentage of units would no longer be very economical. It could even be at times entirely uneconomical and thus lose all interest.
The second problem is that such plates when thereafter found in watches or other timepieces, become elements which only increase the number of the watch components and take up space without having any further utility.
The purpose of this invention is to bring about a solution to these two problems.